60 research outputs found
Luminosity-dependent spectral and timing properties of the accreting pulsar GX 304-1 measured with INTEGRAL
Context: Be/X-ray binaries show outbursts with peak luminosities up to a few
times erg/s, during which they can be observed and studied in
detail. Most (if not all) Be/X-ray binaries harbour accreting pulsars, whose
X-ray spectra in many cases contain cyclotron resonant scattering features
related to the magnetic field of the sources. Spectral variations as a function
of luminosity and of the rotational phase of the neutron star are observed in
many accreting pulsars. Aims: We explore X-ray spectral and timing properties
of the Be/X-ray binary GX 304-1 during an outburst episode. Specifically, we
investigate the behavior of the cyclotron resonant scattering feature, the
continuum spectral parameters, the pulse period, and the energy- and
luminosity-resolved pulse profiles. We combine the luminosity-resolved spectral
and timing analysis to probe the accretion geometry and the beaming patterns of
the rotating neutron star. Methods: We analyze the INTEGRAL data from the two
JEM-X modules, ISGRI and SPI, covering the January-February 2012 outburst,
divided in six observations. We obtain pulse profiles in two energy bands,
phase-averaged and phase-resolved spectra for each observation. Results: We
confirm the positive luminosity-dependence of the cyclotron line energy in GX
304-1, and report a dependence of the photon index on luminosity. Using a
pulse-phase connection technique, we find a pulse period solution valid for the
entire outburst. Our pulse-phase resolved analysis shows, that the centroid
energy of the cyclotron line is varying only slightly with pulse phase, while
other spectral parameters show more pronounced variations. Our results are
consistent with a scenario in which, as the pulsar rotates, we are exploring
only a small portion of its beam pattern.Comment: 12 pages, 12 figures, Accepted for publication in A&
A0535+26 in the April 2010 outburst: probing the accretion regime at work
A number of accreting X-ray pulsars experience spectral changes, both on the
long time scales and on the time scales of the neutron star spin period. The
sources seem to form two distinct groups which differ by the type of the
spectral variations with flux. Such a bimodality probably reflects two
different regimes of accretion that may result in a particular pulsar depending
on its luminosity - so-called sub- and super-critical regimes. We investigated
the behavior of the spectral parameters of the Be/X-ray binary system A
0535+26, as a function of flux and pulse phase. We used the data collected with
INTEGRAL and RXTE during the April 2010 outburst of the source. We analyzed the
phase-averaged and phase-resolved spectra and performed pulse-to-pulse spectral
analysis of the pulsar. Our analysis reveals variability in the continuum
parameters of the pulse-averaged spectrum of the source with flux. The
pulse-averaged cyclotron line energy does not change with the source luminosity
during the outburst, which is consistent with previous studies. Our pulse-phase
resolved and pulse-to-pulse analyses reveal, however, indications for a
positive correlation of the cyclotron line energy with flux, as well as a
flux-dependence of the continuum parameters. Based on the observed behavior, we
argue that A 0535+26 operates at the sub-critical accretion regime.Comment: 9 pages, 11 figures, accepted for publication in A&
Evidence for different accretion regimes in GRO J1008-57
We present a comprehensive spectral analysis of the BeXRB GRO J1008-57 over a
luminosity range of three orders of magnitude using NuSTAR, Suzaku and RXTE
data. We find significant evolution of the spectral parameters with luminosity.
In particular the photon index hardens with increasing luminosity at
intermediate luminosities between erg s. This
evolution is stable and repeatedly observed over different outbursts. However,
at the extreme ends of the observed luminosity range, we find that the
correlation breaks down, with a significance level of at least . We
conclude that these changes indicate transitions to different accretion
regimes, which are characterized by different deceleration processes, such as
Coulomb or radiation breaking. We compare our observed luminosity levels of
these transitions to theoretical predications and discuss the variation of
those theoretical luminosity values with fundamental neutron star parameters.
Finally, we present detailed spectroscopy of the unique "triple peaked"
outburst in 2014/15 which does not fit in the general parameter evolution with
luminosity. The pulse profile on the other hand is consistent with what is
expected at this luminosity level, arguing against a change in accretion
geometry. In summary, GRO J1008-57 is an ideal target to study different
accretion regimes due to the well constrained evolution of its broad-band
spectral continuum over several orders of magnitude in luminosity.Comment: 13 pages, 7 figures, 3 tables. Accepted for publication in A&
Discovery and modelling of a flattening of the positive cyclotron line/luminosity relation in GX 304−1 with RXTE
The Rossi X-ray Timing Explorer (RXTE) observed four outbursts of the accreting X-ray binary transient source, GX 304−1 in 2010 and 2011. We present results of detailed 3–100 keV spectral analysis of 69 separate observations, and report a positive correlation between cyclotron line parameters, as well as other spectral parameters, with power-law flux. The cyclotron line energy, width and depth versus flux, and thus luminosity, correlations show a flattening of the relationships with increasing luminosity, which are well described by quasi-spherical or disc accretion that yield the surface magnetic field to be ∼5 × 10^(12) Gauss. Since HEXTE (High Energy X-ray Timing Experiment) cluster A was fixed aligned with the Proportional Counter Array field of view and cluster B was fixed viewing a background region 1°.5 off of the source direction during these observations near the end of the RXTE mission, the cluster A background was estimated from cluster B events using
HEXTEBACKEST.
This made possible the detection of the ∼55 keV cyclotron line and an accurate measurement of the continuum. Correlations of all spectral parameters with the primary 2–10 keV power-law flux reveal it to be the primary driver of the spectral shape. The accretion is found to be in the collisionless shock braking regime
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